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Abstract1. Several theories aim at predicting changes in the interactions between plants and herbivores over the lifetime of a plant. Hypotheses based on ontogenetic changes in resource allocation to plant defence and in plant apparency lead to partly opposing predictions regarding the differences in levels of herbivory between juvenile and mature plant individuals.
2. We tested these predictions by measuring background foliar losses to insects in saplings and mature trees of downy birch (Betula pubescens) in ten sites along a latitudinal gradient from 60 to 69ºN in boreal forests of Northern Europe.
3. The percentage of consumed leaf area increased, and the variation in the levels of herbivory among plant individuals decreased, for tree sizes ranging from small saplings (2−12 cm tall) to large saplings (13−80 cm tall) and then to mature trees (3−18 m tall).
4. Small saplings had higher foliar quality for insects, as indicated by better performance of leafmining larvae of Eriocrania semipurpurella and by greater specific leaf area, compared with large saplings and mature trees. The average percentage of leaf area consumed from a damaged leaf, which reflects the inducibility of local defence responses to insect damage, did not vary among the birch size classes.
5. The foliar losses to insects decreased nearly fivefold with an increase in latitude from 60 to 69ºN, and the relative differences in these losses among the birch size classes were independent of latitude.
6. Our findings fit well with the predictions based on a plant apparency hypothesis, but do not support predictions based on ontogenetic changes in resource allocation to plant antiherbivore defences. We conclude that the generally overlooked effects of apparency on plant damage by insects can explain, at least in some cases, the frequently observed lack of correspondence between the levels of plant defences and herbivory.